The present invention relates to a projection display. Such a display may be in the form of a compact convertible projector for displaying enlarged images from conventional direct view spatial light modulators, such as liquid crystal devices, for multiple viewers. Displays of this type have uses in portable office equipment, desktop office equipment, television equipment and display presentations.
U.S. Pat. No. 5,629,806 discloses a display system for providing private viewing and for displaying a relatively large image from a small direct view image source. The system comprises an image source such as a cathode-ray tube, electro-luminescent display or direct view liquid crystal display (LCD), focusing, conjugating and folding optics. The conjugating optics include a retro-reflector and a beamsplitter.
U.S. Pat. No. 5,418,584 discloses a retro-reflective array projection screen for displaying virtual images. The screen comprises a large collimating element and an array of small retro-reflective elements for reflecting the projected image back to an external exit pupil. The retro-reflective elements are of the diffractive type with a rear mirror for reflecting light back through the diffractive element. This system is also provided for private viewing.
FIG. 1 illustrates a known type of overhead projector of the reflection type for images fixed on transparencies. A projection light source including a condensing optic 1 illuminates a transparency 2 which is placed on a reflective Fresnel lens 3. The axis of the lens 3 is laterally spaced from the axis of the condensing optic 1 so that the lens 3 images the illuminator at a projection lens 4, which is laterally spaced from the condensing optic 1. A folding mirror 5 directs light onto a projection screen (not shown).
U.S. Pat. No. 5,132,823 discloses a multipurpose LCD suitable for use as a reflective display and overhead projection panel. The LCD comprises a liquid crystal layer which is PIXELLATEd with the picture elements (pixels) being switchable between high and low scattering states. The LCD is disposed on top of a removable corner cube retro-reflector. The retro-reflector is used to improve the image contrast.
U.S. Pat. No. 5,353,075 discloses an arrangement which is convertible between direct view operation and overhead projection operation. For direct view operation, an LCD is disposed over a backlight. For projection operation, the LCD is used in place of a transparency in a conventional projection display.
U.S. Pat. No. 5,668,695 discloses a portable computer whose main body and lid are linked together and which may be used with a conventional overhead projector.
U.S. Pat. No. 5,593,221 discloses the use of an LCD as a projection transparency for a conventional type of overhead projector.
Engberg S. J., xe2x80x9cHolographic techniques change shape of retro-reflectorsxe2x80x9d, Euro Photonics, December/January, 1998, p 37-38 and Hardin R. W., xe2x80x9cDiffraction brings street signs to lifexe2x80x9d, Photonics Spectra, December, 1997, p 40 disclose broadband diffractive retro-reflectors in the form of diffractive Fresnel zone plates for creating light sources on the front surface of a retro-reflector with textured back reflective surface to produce semi-Lambertian scatter to improve the acceptance angle.
U.S. Pat. No. 5,515,354 discloses the use of a diffractive retro-reflector in an optical pick up in the form of a blazed diffraction grating with a reflective film on its rear surface.
U.S. Pat. No. 5,801,793 discloses an LCD which has a removable backlight so that the LCD may be used in direct view and projection modes.
Gallagher T., xe2x80x9cStandard registration mark-pleasexe2x80x9d, Holography News, vol. 11, no. 5, p. 4, 1997 discloses the use of a holographic retro-reflection registration mark for use in precise positioning of embossed holograms in the printing industry. In particular, an embossed transparent plastic hologram having a rear metal reflecting layer is disclosed.
Ralli P. J. and Wenyon M. M., xe2x80x9cImagix((trademark)) holographic diffusers for reflective liquid crystal displaysxe2x80x9d, SID 96 disclose the use of a holographic reflector made of a photopolymer with an STN backlit display device to allow such a device to be used with ambient overhead illumination. FIG. 2a illustrates such an arrangement in the direct view mode whereas FIG. 2b illustrates such a system in the reflection mode.
As shown in FIG. 2a, a transmission mode LCD 10 is disposed above a holographic reflector 11, which is disposed above a backlight 12. In the backlight mode, the backlight 12 is illuminated and directs light 16 through the holographic reflector 11, which has no visible effect, and the LCD 10 towards an observer whose eye is shown at 14.
In the reflection mode illustrated in FIG. 2b, the LCD 10 is illuminated from a suitable light source to provide the overhead illumination 13 within a predetermined acceptable angle of the holographic reflector 11. Provided the illumination occurs within the acceptance angle; the holographic reflector 11 acts as a reflector and directs diffracted light 18 back through the LCD 10 towards the eye 14 of the observer. The holographic reflector 11 may also work in association with a diffuse rear metallic reflector 17.
FIG. 3 illustrates a conventional projection display using a transmission mode LCD 10. An illuminator comprises a light emitter in the form of a lamp 20 and collecting optics shown as a parabolic mirror 21. The resulting collimated light beam is supplied to an homogeniser comprising a first homogeniser lens array 22 and a second homogeniser lens array 23. Light from the homogeniser is passed to an array of polarisation beam splitter cubes and half-waveplate strips 24, a first condensing optic 1 and a second condensing optic 25.
Light from the light source illuminates the LCD 10 and is modulated by the displayed image. The output light is supplied to a projection lens 4 which projects an enlarged image onto a front-projection or back-projection screen (not shown). The collecting optics 21 illuminate the first homogeniser lens array 22 with collimated light from the lamp 20. The array 22 produces an image of the light source formed by the lamp 20 and the collecting optics 21 at each of the lenses of the second homogeniser lens array 23. The lens array 23 and the first condensing optic 1 produce an image of each of the lenses of the array 22 at the plane of the LCD 10. The polarisation recirculation cubes in conjunction with the array of half-waveplates 24 convert the polarisation so that all of the light supplied to the LCD 10 is of the same linear polarisation. Images of the array 22 at the plane of the LCD 10 are overlaid by means of the first condensing optic 1. The second condensing optic 25 forms an image of the light source at the entrance pupil of the projection lens 4, which images the LCD 10 at the screen.
FIG. 4 illustrates a known type of projection display using telecentric imaging to avoid the need for a field lens such as 25. The display shown in FIG. 4 uses a reflection-mode LCD 10 provided with a rear metallic plane reflector internal to the liquid crystal layer and has a folded optical path provided by a turning beam splitter 26 which may be a polarising beam splitter.
The illuminator shown in FIG. 4 is of the same type as shown in FIG. 3. Light from the condensing optic 1 is reflected by the beam splitter 26 onto the LCD. Light passes through the LCD in accordance with the modulation by the displayed image and is reflected back through the LCD 10 by the rear metallic reflector. However, the output light from the LCD has a greater spread than systems based on field lenses and, after passing through the beam splitter 26, requires that the projection lens 4 have a greater input aperture size than the size of the LCD.
U.S. Pat. No. 5,663,816 discloses an arrangement which is similar to that shown in FIGS. 2a and 2b of the accompanying drawings.
WO 95/12826 discloses a reflective liquid crystal display which is illuminated by ambient light. The display has a rear holographic reflector which redirects the diffracted light away from the specular reflection direction so as to improve display brightness.
U.S. Pat. No. 5,389,982 discloses a projection display which uses three liquid crystal devices in reflection mode for modulating the three primary colours. A single light source illuminates a beamsplitter which splits the light spectrum into the three colours and directs the light to the liquid crystal displays. The reflected light then passes back through the prism to a projection system.
U.S. Pat. No. 5,321,789 discloses a reflective liquid crystal display of the projection type. In one embodiment, the liquid crystal display has a front fibre plate, presumably for reducing parallax errors. The angle of incidence of light from the light source is equal to and opposite the angle of reflection to the projection optics.
JP 0 928 1477 discloses a direct view reflective liquid crystal display. A front hologram performs colour filtering to direct red, green and blue light to the appropriate pixels. The display has a reflective rear hologram.
According to the invention, there is provided a projection display comprising a transmissive spatial light modulator and an illuminator for illuminating the modulator from the front thereof, characterised by a hologram disposed on the rear of the modulator for imaging and reflecting back through the modulator light from the illuminator received through the modulator.
The hologram may be arranged to form an image of part of the illuminator in front of the modulator.
The hologram may be arranged to perform the functions of a reflector and an off-axis lens with aberration correction.
The hologram may be a volume reflection hologram.
The hologram may include at least one continuous region for reflecting and imaging light of respective primary colours.
The at least three regions may be continuous layers.
The illuminator may include a light source and a first condensing optic for forming an image of the light source at a first location.
The hologram may be arranged to form an image of the light source at a second location which is spaced from the first location.
The second location may be laterally spaced from the first location.
The first and second locations may be laterally offset with respect to the modulator.
The display may include a projection optic for forming an image of the modulator on a screen.
The projection optic may have an entrance pupil disposed substantially at the second location.
The illuminator may include a field stop associated with the first condensing optic and a second condensing optic for imaging the field stop on the modulator.
The hologram may be pixellated to correspond with the pixellation of the spacial light modulator, and hence reflect light of the corresponding SLM pixel primary.
The display may include a turning optic for turning light from the illuminator towards the modulator.
The turning optic may include a half mirror.
The turning optic may include a reflecting surface.
The reflecting surface may be arranged to turn light from the modulator.
The first and second locations may be disposed substantially at respective adjacent surfaces of the prism.
The illuminator may include an homogeniser disposed between the light source and the first condensing optic.
The illuminator may include a polarisation conversion optic disposed between the homogeniser and the first condensing optic.
The light source may include at least one light emitter and collecting optic.
The or each collecting optic may include a mirror.
The light source may include respective primary colour emitters.
The modulator may include a liquid crystal device.
The device may include a rear substrate, a liquid crystal layer, and a rear polariser disposed between the rear substrate and the liquid crystal layer, the hologram being disposed between the liquid crystal layer and the rear substrate.
A glass layer may be disposed between the liquid crystal layer and the hologram.
The modulator may include a rear polariser.
The rear polariser may include first and second polarisers and the hologram may be disposed between the first and second polarisers.
The display may include a third polariser for supplying incident light with a first polarisation from the illuminator to the modulator and a fourth polariser for passing light reflected from the modulator with a second polarisation substantially orthogonal to the first polarisation.
The display may include a polarising beam splitter for reflecting incident light with a first polarisation from the illuminator to the modulator and for passing light reflected from the modulator with a second polarisation substantially orthogonal to the first polarisation.
The modulator may include a front polariser and waveplate.
The modulator may include a plurality of picture elements, each of which is switchable between a light transmissive mode and a light scattering mode.
The display may include a backlight for illuminating the modulator through the hologram to provide a direct viewing display mode.
It is thus possible to provide a projection display which makes use of a transmissive spatial light modulator such as an LCD and which reduces or overcomes disadvantages of known displays of this type. The hologram can be recorded in such a way that it performs an imaging function which is compensated at least partly for aberrations so as to provide improved display quality. For instance, first order correction for aberrations during recording can provide a substantially uniform reflection with uniform illumination across the whole image. Also, the hologram may be relatively thin so that parallax errors are substantially uniform across the image and illumination uniformity is maintained.
It is also possible to provide a projection system using a relatively large LCD panel with a compact illumination system. This has the advantage of allowing a relatively large pixel size at high resolutions, thus maximising the aperture ratio of the individual pixels. Hence it is possible to produce a compact, high efficiency projection system. Unwanted diffraction effects from very small pixels that are found in high resolution small panels are minimised. The holographic field element gives a flat image plane of the LCD so that the Modulation transfer function (MTF) of the projection system is maintained.
The projection lens in such a system is a low numerical aperture, wide field angle lens as opposed to a high numerical aperture low field angle lens. This allows the lens to be of small input aperture size, reducing stray light effects in the display and improving display contrast.
A compact folded projection system can be produced from such a large panel display. Conventional systems require bulky optics for large panels, but the use of this kind of reflection geometry reduces the system bulk substantially.
The hologram may be recorded under conditions to match the system etendue. The illuminator dimensions determine the system etendue, which is also proportional to the product of the modulator area and the solid angle of modulator illuminating light. A given modulator area therefore results in a predetermined illumination numerical aperture for optimum light throughput. This also determines the system aberrations and thus the working distance of the hologram. The abberations of the hologram may be tuned so that the image of the light source produced by the hologram is substantially uniform at each point on the hologram.
The hologram may have substantially no visible structure. Accordingly, there is no Moire beating with the structure of the modulator so that no undesirable visible fringes are produced.
By using a reflection volume hologram, the hologram only reconstructs at or near the conditions determined at manufacture of the hologram. Any other illumination structure is substantially unaffected. Accordingly, the display may be used in a direct view mode with a backlight and without modification to the modulator. In particular, it is not necessary to remove the hologram in order to provide the direct view mode.